The present invention concerns apparatus and methods for the shielding (absorption) of incident gamma radiation and its conversion to less energetic photons. Gamma photons (often denoted by the Greek letter gamma, γ) are a form of very high energy, very high frequency, very short wavelength, and very penetrating electromagnetic radiation emitted by, for example:                Atomic nuclei making a transition from an initial excited nuclear state to a subsequent lower energy state, or        Transmutation reactions (nuclear reactions in which one element changes to another) in which prompt gamma rays are emitted by an atomic nucleus after it has absorbed a type of neutral elementary particle called a neutron, or        Radioactive decay of nuclei in processes such as fission of unstable radioactive isotopes (e.g. Uranium-235, used in nuclear weapons and some commercial nuclear power plants), and        Other subatomic processes involving elementary particles such as electron-positron annihilation.        
Gamma photons, alternatively called “gamma rays”, “gamma radiation” or “gamma emissions”, have the following basic characteristics:                Energy: >100 keV        Frequency: >1020 Hz        Wavelength: <10−13 meters        Penetrating Power: much more penetrating than X-rays in normal solid materials; gammas are the most powerful form of electromagnetic radiation.        
The following terminology, consistent with accepted practice, will be used in this Invention:                Infrared (IR)—associated with the concept of “heat” because it emitted as radiation by hot objects; efficiently absorbed by many solid materials. IR is a broad spectral segment; photon energies range from 0.0012 eV up to ˜1.65 eV; this includes the infrared, intermediate infrared, and far infrared bands.        Visible Light—electromagnetic radiation seen by the human eye comprises a relatively narrow slice on the entire electromagnetic spectrum with photon energies that range from 1.65 to 3.1 eV.        Ultraviolet (UV)—broadly defined, photon energies range from 3.1 to 124 eV.        Extreme Ultraviolet (EUV)—in optical and laser work, this term commonly refers to photons with energies that range from roughly 89 eV up to about 113 eV. This range narrowly overlaps the lower boundary of soft x-rays.        Soft X-Rays—photon energy ranges from 0.10 keV (100 eV) up to 10 keV        Hard X-Rays—photon energy ranges from 10 keV up to 1,000 keV (1 MeV)        Gamma Rays—generally refers to any electromagnetic photon with an energy>100 keV; please note that this accepted definition overlaps that of “hard X-rays” from 100 keV to 1,000 keV (1 MeV)—this overlap is alternatively called “Soft Gamma Rays”. In the narrowest possible definition, the energy of gamma ray photons ranges from 1 MeV up to whatever. Gamma photon energies can get extremely high: gamma rays emitted in a 1996 burst recorded from an astronomical object named Markarian 421 (known as a “blazar”) reached 300 GeV—astronomers believe that they can reach TeVs.        Prompt or Activation Gamma Rays—these terms imply gamma rays that are emitted from an atomic nucleus either: (a.) within one (1) nanosecond after a neutron absorption (“capture”) event, or (b.) within a few nanoseconds after the instant of nuclear fission. Gammas arising from neutron captures on nuclei can be described either as “prompt gammas” or as “activation gammas”. Gammas that are emitted contemporaneously with fission events are typically referred to as “prompt gammas.”        Delayed or Decay Gamma Rays—in the case of neutron capture on nuclei without inducing fission, the terms “delayed” or “decay” gamma rays generally imply gammas that arise in the course of subsequent decay of more neutron-rich, unstable product(s) that are produced by the initial neutron capture event. Analogously, in the case of a fission event (whatever the proximate cause), the terms “delayed” or “decay” gammas generally mean gammas that arise from the later (depending on their half-lives) radioactive decay of the fission product(s)/fragments.        
The relative position and key characteristics of gamma rays compared to the rest of the electromagnetic spectrum can be seen in the following Table 1:
TABLE 1Gamma Rays and the Electromagnetic SpectrumWavelengthFrequencyEnergy per PhotonName of Wave(m = meters)(Hz)(eV)AM Radio10210610−9FM, TV 110810−7Radar10−110910−6Microwaves10−2101010−5Infrared10−5101310−2Visible Light10−71015 1Ultraviolet10−81016101X-Rays10−101018103Gamma Rays10−131021106
The data shown above in Table 1 is expressed graphically in FIG. 1 in which the relative positions of the infrared bands, soft X-rays, and soft/hard gamma rays are shown versus the entire electromagnetic spectrum.
FIG. 2 shows the relationship between gamma rays and the rest of the electromagnetic spectrum on the basis of wavelength; it also illustrates the relatively small percentage occupied by the visible segment of the entire spectrum.
Gamma photons interact with normal matter through three main processes (Photoelectric Effect, Compton Effect, and pair production); in order of increasing energy of incident gamma photons required to initiate them, they are as follows:                Photoelectric Effect (gamma photon energy≦˜0.5 MeV)—an incident gamma ray photon strikes and is fully absorbed by an atomic electron, which is then subsequently ejected from the atom as an energetic photoelectron, leaving an ionized atom. The gamma photon is now gone, having been absorbed completely by one atomic electron. This process is graphically illustrated in FIG. 3.        Compton Scattering (˜0.5 MeV≦gamma photon energy≦˜5.0 MeV)—in this process, an incident gamma ray photon strikes an atomic electron, ionizing it, and giving up part of its energy to the struck electron, which is then ejected from the atom as an energetic Compton electron. This process is graphically illustrated in FIG. 4. After scattering, the incident gamma photon, now at a lower level of energy, continues through the material or medium in which the atom on which it scattered was located. Depending upon its remaining energy, this lower-energy gamma photon may interact further with materials in its subsequent path via the Photoelectric Effect, Compton scattering, or pair production.        Pair Production (mostly at gamma photon energies≧˜5.0 MeV)—in this process, an incident gamma ray photon makes it all the way through the outer cloud of atomic electrons and encounters the intense internal Coulomb field of a atomic nucleus. While in the grip of the nuclear Coulomb field, if an incident gamma photon's energy is greater than a minimum value of 1.022 MeV, pair production may occur. In this process shown in FIG. 5, the entire energy of an incident gamma photon (which then disappears) is converted into an electron-positron (positive electron) pair that fly-off in opposite directions. When such an emitted positron slows down and encounters an ordinary electron, they annihilate each other: two 0.511 MeV gamma photons are produced, which can subsequently interact with matter via the Photoelectric or the Compton Effect.        
Relative contributions of each of process as a function of a “target” atom's nuclear charge (Z or atomic number of absorber) versus incident gamma photon energy (MeV) is shown in FIG. 6. In general, the Photoelectric Effect becomes dominant at low gamma ray energies. In the middle range of gamma ray energies from ˜1-5 MeV, the Compton Effect dominates over the Photoelectric Effect and pair production. At very high energies, the Photoelectric Effect is absent and pair production increasingly dominates over the Compton Effect.
FIG. 7 illustrates high-energy photon absorption cross sections (measured in barns=b) versus photon energy for a relatively low-Z material (Carbon, Z=6); FIG. 8 shows the same data for a relatively dense, high-Z material (Lead, Z=82). Note the cross sections for absorption of gamma radiation between ˜100 keV and 10 MeV for Carbon versus Lead.
In FIGS. 7 and 8, examination of the top line on the charted data denoting “Total” photon absorption cross-section per atom illustrates how a high-Z material (e.g. Lead) can be a substantially better gamma shield (i.e. photon absorber or attenuator) than a relatively low-Z material (e.g. Carbon) for gamma photons at energies from 100 keV to ˜10 MeV. Implicit in this comparison is the fact that electron density is much higher in high-Z materials such as Lead than in low-Z materials such as Carbon.
It is important to note that in planetary environments, typical local (non stellar) gamma sources (natural, man-made, or man-induced) have photon energies that mostly range from 100 keV to ˜10 MeV. Gammas produced in nuclear fusion processes can easily reach ˜20+ MeV, but are rarely encountered on the surfaces of planets with atmospheres, including Earth. Thus, in most environments, the gamma photon energy range of ˜0.5 MeV to ˜10 MeV includes a very substantial portion of the gamma ray fluxes that might conceivably be encountered.
Gamma Photon Radiation Damages Biological Organisms and Modern Electronic Systems:
As shown in FIG. 9, exposure to electromagnetic (EM) radiation has various effects on biological and electronic systems. Compared to the rest of the EM spectrum, X- and gamma-rays can cause particularly severe, long term damage to various types of matter found in solid structures. Comparatively high potential for damage occurs from exposure to external gamma radiation because gamma photons have higher energy per photon, thus high frequencies, short wavelengths and tremendous penetrating power compared to charged particles such as alphas and betas (energetic electrons). X- and gamma-rays are thus able to ionize or otherwise excite a variety of atoms and/or molecules located almost anywhere deep inside a living organism or unshielded electronic component.
A simplified example of the process of ionization is illustrated in FIG. 10. What happens is that a high energy photon (anything>30 eV) first hits an electron of an atom located somewhere within a biological structure or electronic component. That impact can knock an electron out of the atom, leaving behind a positively charged atom (ion) and an energetic electron (together forming an “ion pair”), both of which can interact further with other local atoms or molecules; this is essentially what happens in both the Photoelectric Effect and Compton scattering (discussed earlier).
Types of Damage to Living Organisms from Incident Gamma Radiation:
The following illustrate various types of damage from exposure of unshielded living organisms to external gamma radiation; its penetrating power enables it to irradiate 100% of the materials comprising most organisms (99% are low-Z elements: H—60%; O—25%; C—12%; and N—5%):                Internal production of large quantities of highly reactive, damaging chemical species called “free radicals” that can wreak havoc in biochemical reaction networks by reacting extensively and uncontrollably with other molecules,        Disruption of molecular chemical bonds resulting in “dangling bonds” and unnaturally activated macromolecules that can disrupt biochemical pathways,        Production of new chemical bonds between nearby molecules, or creation of unnatural cross-linkages within various types of high molecular weight macromolecules, thus altering their biological function or biochemical activity,        Deleterious alteration of the structures of biological macromolecules that are specifically involved in many critical cellular processes, e.g. proteins (structural and enzymes), RNA, and DNA.Major Types of Damage to Modern Electronic Equipment from Gamma Radiation:        
Similar to biological systems, many types of electronic equipment upon which modern civilization depends can be very sensitive to ionizing EM radiation (gammas and X-rays of terrestrial or extraterrestrial origin) and extremely energetic charged particles traveling at nearly the speed of light (cosmic rays of extraterrestrial origin of which 89% are protons, 10% are helium nuclei, and 1% are nuclei of many other elements) that are able to penetrate surrounding metallic enclosures.
Examples of essentially ubiquitous, potentially radiation-sensitive electronic components include microprocessors, computer memory chips, and many other types of integrated circuits that have micron- or sub-micron-sized features. Gamma radiation damage to modern electronic devices falls into two major categories: ionization effects and lattice displacements.
Damage to Electronics from Ionization Effects:
This is probably the most important class of damage created by irradiation of electronic components with gamma photons and/or hard X-rays. Key damage parameters include the total “dose” and energies of gamma photons, dose rate, and specific locations of ionization interactions within an electronic device.
Ionization-related damage can occur as a result of an interaction between a semiconductor device or integrated circuit's lattice atoms and a combination of atomic ions and energetic electrons (effectively beta particles) produced via the Photoelectric Effect and/or Compton scattering of gamma photons within device materials. Such secondary internal interactions between gamma photons and device materials (resulting from radiation passing through cases, circuit boards, components, and devices) are termed systems-generated electromagnetic pulse (SGEMP) effects.
Examples of several different types of gamma-induced ionization effects include                Photocurrents—Gamma irradiation can generate electron-hole pairs in semiconductor materials (a 1.0 MeV gamma photon can generate 105 electron-hole pairs/photon); such free carriers generate photocurrents as they move through depletion regions of p-n junctions of integrated circuits. The size of these currents can be orders of magnitude greater than normal system levels and can cause temporary or permanent damage to electronic circuits, depending on specific circumstances. Both CMOS devices and MOSFET transistors are affected by photocurrents; free charge can be permanently trapped in certain oxide materials. In the extreme case of the nearby detonation of a nuclear weapon, large transient photocurrents can be created throughout the entire structure of irradiated semiconductors.        “Glitches” or Soft Errors—being short lived, these fall under the term “transient radiation effects in electronics” (TREE). They are transient, temporary effects that do not cause permanent damage to a device. In this effect, local charge fluctuations can cause transistors to randomly open which can in turn change the logical states of memory cells and flip-flops. Such “error states” are transitory, and disappear. In the case of DRAMs, the effects of such errors can be obviated with error-correction logic designed into the internal circuitry of the chip (ECC memory chips). However, depending on the size of the transient, this effect can also lead to permanent changes if there is actual junction burn-out or a “latch up”.        “Latchup”—is a particularly destructive ionization effect that can potentially lead to catastrophic failure of electronic devices. This effect involves the creation of a high-current, low-voltage nanoscale pathway within an integrated circuit. Such an unnatural flow of current within a highly ordered device can cause a circuit to temporarily malfunction—in the worst case, a tiny segment of the device can be physically burned-out through joule heating of the material.Damage to Electronics from Lattice Displacements:        
This is a second major class of damage produced by gamma photons on electronic components; it occurs less frequently than ionization damage. Lattice displacement can only occur with relatively high-energy gamma photons which, for most materials and real-world environments, involve gamma photon energies from ˜2 MeV to 10 MeV. This type of damage is produced when an atom in a semiconductor has a Compton scattering interaction with a gamma photon and the “target” atom is physically moved (displaced) from its normal position relative to neighboring atoms in the semiconductor's 3-D lattice structure. Such displacements can cause permanent physical changes in lattice structure (e.g. various types of structural defects) and drastically alter the properties of materials (e.g. changes in bandgap energy levels), depending upon the total “dose” and dose rate of gamma photons as well as the precise locations of resulting lattice defects.
Specialized Niche Markets for Radiation-Hardened Electronic Components:
Although small compared to the size of the global markets for commercial off-the-shelf (COTS) electronic components, there are specialized commercial markets for radiation-hardened (rad-hard) electronic devices. These markets serve primarily military customers with defense systems that must be able to operate either out in space and/or in combat environments in which nuclear or thermonuclear detonations can potentially occur. “Rad hard” integrated circuits and electronic systems for military and space applications are substantially more expensive than ordinary commercial products. Many of the essential features of this unique market segment are summarized in quotes from an employee of a company, Maxwell Technologies that is well known for its product lines of rad hard components and systems:                “The trouble is, building “hardened by design” electronics takes years, the costs are high, and at the end, the systems are often out of date with the current commercial state-of-the-art. Instead, there is an increasing need to use commercially-based computer systems in space to provide reliable, low power operation with throughput performance that is orders of magnitude higher than would be achievable with current state-of-the-art radiation-hardened flight systems.”        “Traditionally, space-qualified single board computers (SBCs) have trailed the cutting-edge commercial and military products by factors of performance—often times in excess of 10-100×. Much of this discrepancy is due to the difficulty in radiation hardening, at either the component or system level, that is required for a product to survive and operate in the harsh environment of space.”        “A simple and direct way to extend the life of semiconductors is to shield them against the ionizing dose environment. Electrons and protons can be shielded against relatively effectively, while energetic heavy ions and gamma radiation are difficult to shield against. Therefore, such an approach is attractive for space systems in which electrons and protons make ionizing dose a threat. While simple and direct, there is a science to shielding if one is to use it to the best advantage. The ability to use shielding is dependent on having enough information about the device to be shielded, as well as the intended environment.”        “In addition to natural space radiation, some military systems must be protected from ionizing radiation from man-made sources in space, such as nuclear event-generated [nuclear weapons explosions] X-rays [and gamma rays]. Shielding can also prove to be very effective in reducing X-ray effects, depending on the energy of the photons. The problem of shielding against X-rays is somewhat different from that of shielding against the natural environment.”        “The first such difference is found in the directionality of the radiation. Natural space radiation is, to a good approximation, omni-directional, while man-made radiation comes from a “point source,” and is, therefore, highly directional. Simple slab shields are rather effective in dealing with omni-directional space radiation but are insufficient against X-rays [and especially even higher energy gamma rays]. Because the source of radiation can't be predicted, the device must be shielded in all directions in order for shielding to be effective. Such distinctions must be made clear to users interested in using shielded packages to assure that, in the drive to switch to commercial components, users do not lose essential elements of radiation protection.”        “Putting component- and system-level radiation strategies together allows using modern COTS processors such as the wildly popular PowerPC processor architecture from Apple, IBM and Motorola in space-based SBCs.”        
The source of the above quotes is a published article titled, “An Integrated Approach with COTS Creates Rad-Tolerant Single Board Computer for Space”, Chad Thibodeau, Product Manager, Maxwell Technologies, COTS Journal, December 2003.
Natural Background of Terrestrial Gamma and Hard X-Ray Radiation:
Earth's thick atmosphere protects the planetary surface from most gamma radiation originating in space from various sources located outside the solar system. However, the surface of the earth and its environs have always experienced a small, natural flux of background gamma and hard X-ray radiation, most of it being produced by decay chains of various radioactive isotopes found naturally in the earth's crust. A few radioisotopes (e.g. 238U) have been around essentially since the planet's formation from the protosolar nebula ˜4 billion years ago. Examples of commonly occurring, gamma-emitting radioisotopes with very different half-lives include:                Bismuth: 212Bi (half-life=˜1 hour) in 228Th decay chain; and 214Bi (half-life=˜20 minutes)        Lead: 212Pb (half-life=˜11 hours) and 214Pb (half-life=˜27 minutes) in 228Th decay chain        Potassium: 40K (half-life=˜1.3 billion years) distribution is ubiquitous; can be found in bananas        Thallium: 208Tl (half-life=˜3 minutes) in thorium-228 (228Th) decay chain        Uranium: 238U (half-life=˜4.5 billion years) weakly radioactive; found in depleted uranium munitions used by the military        
FIGS. 11, 12, 13, and 14 illustrate examples of fairly typical background gamma radiation spectra collected at different locations on the earth's surface, outdoors as well as inside a building structure housing a scientific laboratory. Please note that certain gamma lines in these spectra come from man-made “contaminant” radioisotopes, e.g. 60Co (Cobalt-60, which has a half-life of ˜5.3 years) and 137Cs (Cesium-137 which has a prominent spectral “line” at an energy of ˜660 keV and half-life of about 33 years). The 137Cs line is particularly prominent in FIG. 12 (a soil sample taken from rural Belgium). Thanks to previous nuclear weapons testing in the atmosphere up until the test ban treaty, ongoing commercial reprocessing of spent nuclear reactor fuel in some countries and the Chernobyl nuclear reactor disaster in Russia in 1986, 137Cs temporarily enjoys a nearly ubiquitous distribution across the earth's surface.
Nuclear Weapons, the Nuclear Power Industry, and Commercial Use of Gamma-Emitting Isotopes:
Beginning around the end of World War II, mankind's development and deployment of nuclear weapons (fission and fission-fusion) and closely related nuclear power generation technologies ushered in a new era in which radioisotopes that had been absent from the earth's surface for billions of years were suddenly being created and disseminated in terrestrial ecosystems.
Two atomic attacks on Japan, as well as subsequent nuclear weapons testing in the atmosphere from Jul. 16, 1945 to Nov. 4, 1962 (when atmospheric testing was finally banned) by a number of countries, injected a wide variety of gamma-emitting radioactive isotopes into the earth's environment. Parallel development of civilian nuclear power industries in many countries, along with intimately related fuel production/reprocessing and waste disposal activities, has resulted in further injections of radionuclides into global ecosystems. This includes thousands of low-level unintended releases, as well as the major disaster at the Chernobyl reactor complex in Russia in 1986. Also, a variety radioisotopes produced in nuclear reactors (e.g. 60Co) began to be used commercially as radiation sources in many industrial and medical applications, creating additional human exposure and used equipment disposal issues.
Existing nuclear fission and fission-fusion weapons characteristically produce a short, very intense flash of gammas and hard X-ray radiation that is closely associated with the explosive event. Weapon detonation creates a radiation “point source” that produces an initial burst of prompt<10 MeV gammas and 2 keV-500 keV X-rays (both inside a pulsewidth<<<100 nanoseconds) rapidly followed by a second, more drawn-out burst of delayed gamma photons (this second flux of photon radiation lasts from 100 nanoseconds to 10 milliseconds) at various energies ranging up to a maximum of ˜10 MeV. This initial burst of electromagnetic radiation comprises roughly 5% of the total yield of a conventional nuclear explosion.
Globally, civilian and government-operated nuclear reactors are primarily used for electric power generation and to a much lesser extent for commercial production of various isotopes. Not surprisingly, the “fuel” rods used various types of modern commercial power reactors contain many of the same fissile isotopes that are found in nuclear weapons. Similarly, nuclear fission reactions occurring in power reactors also produce a mixture of both prompt and delayed gamma rays. Like weapons, energies of gammas and hard X-ray photons produced in power reactors range from 100 keV to ˜10 MeV. However, by contrast, nuclear reaction rates in commercial reactors are tightly controlled, and thus produce a more-or-less continuous flux of gamma radiation, unlike the time-compressed, very intense bursts associated with nuclear weapons.
Since size or weight does not particularly matter in commercial nuclear reactors, shielding against various types of radiation, including gammas, can be massive and extensive. However, delayed gamma radiation emanating from radioisotopes found in spent reactor fuel rods presents a more problematic safety issue than nuclear reactors themselves, since the rods must be physically removed from heavily shielded reactors after their use. Typically, such spent fuel rods and fuel assemblies are “temporarily” stored outside reactor buildings in secure, water-filled “cooling ponds” while awaiting transport for disposal in high-level waste facilities such as the proposed site in Yucca Mountain, Nev., or alternatively in Europe, transport to nuclear fuel reprocessing centers.
FIG. 15 illustrates a delayed gamma spectrum collected from a spent nuclear reactor fuel assembly removed after many hours of operation inside a reactor core. The fuel assembly from which this particular gamma spectrum was collected came from an enriched uranium fueled, pool-type, light-water moderated Brazilian research reactor named IEA-R1. Similar to some of the other gamma spectra, FIG. 15 shows a prominent 137Cs line. FIG. 15 differs from the gamma spectra illustrated in FIGS. 11-14 in that its spectrum is much “hotter”, much more complex, and exhibits many more high-count gamma emission lines. FIG. 15 also differs from the previous Figures of gamma spectra in that: (a.) total count rates are substantially higher for more penetrating gammas with energies>1,000 keV (1.0 MeV), and (b.) the 137Cs line at 660 keV has total counts of ˜20,000 as compared to only ˜1,250 for soil found outdoors in Belgium.
Since WWII, there has been increasing industrial and medical use of various gamma emitting isotopes in the form of sealed radiation sources; each isotope so used produces certain characteristic “lines” in the gamma energy spectrum. Those most commonly used in commercially available equipment are: 60Co (Cobalt-60; half-life=5.3 years), 137Cs (Cesium-137; half-life=33 years), and less frequently, 99mTc (metastable Technetium-99; a nuclear isomer; half-life=6 hours, after which it decays via gamma emission to 99Tc which has a half-life=212,000 years). Less commonly used gamma emitters include: 226Ra (Radon-226; half-life=1599 years), 192Ir (Iridium-192; half-life=74 days), and 85K (Krypton-85; half-life=10.7 years). Gamma emitting isotopes are currently used in a variety of wide-ranging commercial applications that include: irradiation of foodstuffs and postal mail to kill harmful microorganisms; medical uses such as various kinds of cancer therapies; and industrial materials characterization such as gauging of product thicknesses on high-speed production lines.
Over the past decade or so, there has been acrimonious scientific debate and little-publicized research funding by several different governments (including the United States, France, China, Russia, and probably even Japan) aimed toward the possibility of developing a new and very different type of tactical nuclear weapon. In a radical departure from the past, such weapons would not be based upon nuclear fission or fission-fusion reactions; they would be based on controlled (triggered) deexcitation (decay) of certain nuclei from metastable excited isotopic states called nuclear isomers.
Unlike other types of isotopes, nuclear isomers do not involve any change in the number of protons or neutrons in an atomic nucleus. Rather, they are an excited metastable or isomeric state of an atom caused by the excitation of a proton or neutron inside its nucleus. For an atom to decay from such a higher-energy isomeric state to a (more stable) lower-energy non-isomeric ground state, the excited proton or neutron in the nucleus requires a change in its spin in order to deexcite and release its excess energy. Such isomeric decays involve two types of transitions: (a.) emission of gamma photons; (b.) internal conversion in which the energy released is used to ionize the atom.
By convention, isomers of a particular atom are designated by a lower-case letter “m” in several types of equivalent representations (e.g. Co-58m, mCo-58, or 58mCo). For atoms that have multiple distinct isotopic isomers, they can be labeled 2 m, 3 m, etc. Nuclear isomers are scattered throughout the entire ensemble of 2,000+ known isotopes of all the elements currently included in the periodic table. Most isomers are very unstable and decay very rapidly, radiating their excess energy within 10−12 seconds; isomers listed in online nuclear databases are restricted mainly to those which have half-lives of 10−9 seconds or more.
The only known “stable” isomer is 180mTa (Tantalum-180m); it has a half-life of at least 1015 years, and may in fact be stable. When 180mTa decays, it radiates hard X-rays. The 178mHf (Hafnium-178m) isomer has a half-life of 31 years; importantly, it has the highest known excitation energy of any nuclear isomer as well as a reasonably long half-life. When a 178mHf atom decays, it releases all of its excess energy as ˜2.4 MeV of gamma radiation. If all of the atoms contained in one kilogram (˜2.2 pounds) of pure 178mHf could be triggered to decay at exactly the same time, it could release ˜900 MegaJoules of energy in an intense flash of nearly pure gamma radiation; this is equivalent to the energy released in an explosion of a quarter of a kiloton of TNT. With an isomer weapon, there would be no lingering radiation or radioactive fallout after detonation. Besides weapons-related applications, there have been recent discussions within the U.S. Department of Energy (DOE) and DARPA concerning the possibility of using nuclear isomers to develop new, ultra high-performance energy storage technologies.
Two key technological problems that must be solved to harness the potential power of nuclear isomers for both weapons and energy storage applications are: (a) triggering them efficiently in a manner that permits a net energy gain from the point of triggering through decay; and (b) triggering enormous numbers of isomer atoms to decay in unison at exactly the same time. For energy storage applications there is also a third major problem: efficiently absorbing and converting gamma radiation into some other form of energy that can be more readily utilized to do work. DOE convened a scientific panel to determine whether 178mHf production was feasible; the panel answered yes, but at very high cost. To date, none of these problems have been solved. A thorough, nontechnical summary of the recent history and scientific controversy surrounding DARPA-DOE-sponsored nuclear isomer research can be found as follows:                “Scary things come in small packages”        Sharon Weinberger        The Washington Post, Sunday, Mar. 28, 2004, Page W15        http://www.washingtonpost.com/ac2/wp-dyn?pagename=article&contentId=A22099-2004Mar24&notFound=trueThe subject of the development of isomer weapons for the U.S. military has also been discussed at length in a recent book:        Imaginary Weapons: A Journey through the Pentagon's Scientific Underworld         Sharon Weinberger, Editor of Defense Technology International         Nation Books (an imprint of Avalon Publishing Group, New York)        2006-276 pp        
In that regard, for several years the U.S. Air Force quietly conducted preliminary exploratory research on concepts involving aerospace applications for nuclear isomers as follows, but USAF work in this area has been suspended pending resolution of the ongoing debate about triggering:                “Analysis of the application of a triggered isomer heat exchanger as a replacement for the combustion chamber in an off the shelf turbojet”        C. R. Hartsfield, Captain USAF        Master's Thesis: AFIT/GAE/ENY/01M-04 March 2001        Dept. of the Air Force, Air University, Air Force Institute of Technology, Wright-Patterson AFB, Ohio        “Design study of triggered isomer heat exchanger-combustion hybrid jet engine for high altitude flight”        C. E. Hamilton, Captain USAF        Master's Thesis: AFIT/GAE/ENY/02-6 March 2002        Dept. of the Air Force, Air University, Air Force Institute of Technology, Wright-Patterson AFB, Ohio        “Isomer Energy Source in Hybrid Jet Engines for High Altitude Reconnaissance Flight”        C. Hamilton, P. King, and M. Franke (USAF Institute of Technology) Journal of Aircraft (0021-8669), 41, No. 1, January-February 2004, pp. 151-155        “Isomer energy source for space propulsion systems”        B. L. Johnson, Captain USAF        Master's Thesis: AFIT/GAE/ENY/04-M101 March 2004        Dept. of the Air Force, Air University, Air Force Institute of Technology, Wright-Patterson AFB, Ohio        “Isomer heat exchanger combustor replacement for a supersonic ramjet powered vehicle”        J. C. Cox, Ensign USNR        Master's Thesis: AFIT/GAE/ENY/04-J02 June 2004        Dept. of the Air Force, Air University, Air Force Institute of Technology, Wright-Patterson AFB, Ohio        
With the foregoing hypothetical aerospace applications, assuming that net-gain isomer triggering can be made to work, there is one additional major technological issue: low-mass materials that can provide critical, “mass-effective” shielding against gamma radiation.
Today, there is still heated scientific debate as to whether energy-efficient net-gain triggering of nuclear isomers is possible. Earlier experimental work by Prof. Carl Collins (Director, Center for Quantum Electronics, University of Texas at Dallas) et al. suggested that net-gain isomer triggering might be possible using much lower-energy X-rays. However, their work has not been replicated by others and, in the opinion of some scientists, has been totally discredited. Nonetheless, a number of researchers have continued experimental and theoretical work in this poorly understood area. For example, the following recent theory paper lends support to the idea that net-gain triggering of the 178mHf isomer may in fact be possible in certain circumstances. This paper also mentions that a threshold triggering energy of 1.1 MeV for the spin-9, 75 keV isomer in 180mTa has been established. However, with an input energy of 1.1 MeV, i.e. 1,100 keV, and decay output energy of only 75 keV, there is no net energy gain from triggering 180mTa:                “Nuclear structure of 178Hf related to the spin-16, 31-year isomer”        Y. Sun, A. Zhou, G. Long, E. Zhao, and P. Walker        Physics Letters B, 589, pp. 83-88, 2004        
As of this writing, it has been unofficially confirmed in 2006 that an experiment was recently conducted at a national DOE nuclear laboratory that reportedly demonstrated statistically significant acceleration of 178mHf decay with an X-ray trigger. No further details are available, nor has anything yet been published in the open scientific literature about these new results.
External Shielding Against Various Types of Incident Radiation:
As stated earlier, external shielding with various materials is one way to protect living organisms and sensitive electronic equipment from exposure to various types of energetic charged particles, neutrons, and penetrating electromagnetic radiation such as gammas and X-rays. For charged particles such as protons, alphas, betas at moderate energies, comparatively little shielding is required to absorb the radiation completely. The greatest danger from the vast majority of alpha and beta particles is not created by external exposure to them (since shielding against them is almost trivial), but rather when they are somehow transported or produced inside a living organism or a nonliving physical enclosure containing sensitive electronic components.
Examples of external shielding against selected types of incident radiation are as follows:
                Alpha particles (positively charged helium nuclei)—stopped by one thickness of ordinary writing paper; everything except the most energetic alphas are generally stopped by the human skin (its thickness varies from ˜0.5 mm or 0.02″ on the eyelids to ˜4.0 mm or 0.16″ on the palms and soles of the feet).        Beta particles (negatively charged energetic electrons)—completely stopped by 1″ of wood, or 0.25″ of Plexiglas plastic; it takes a beta particle with energy>70 keV to fully penetrate human skin.        Protons (positively charged elementary particles)—are stopped completely by relatively modest thicknesses of mid-Z metals; for example, only 25 microns (˜0.001″) of copper (Cu; Z=29) will completely stop all protons with energies<2.5 MeV. About 25 mm (˜1″) of high-purity germanium (Ge; Z=32) will stop protons with energies up to ˜100 MeV.        Neutrons (no charge; an elementary nuclear particle that is stable only inside of an atomic nucleus—its half-life as a free particle outside of a nucleus is only around 13 minutes, after which it beta decays into a proton, electron, and a neutrino)—absorbed more effectively by hydrogen rich (H; Z=1) materials. Examples include: water, paraffin wax, polyethylene, and/or concrete (17 atom % hydrogen). Unfortunately, many types of materials can be “activated” by neutron absorption and subsequently produce prompt or delayed gammas and radioactive isotopes as a result; thus, gamma shielding may also be required. Neutrons are particularly damaging to living organisms because of their being composed of a high percentage of hydrogen (˜60%) associated with carbon-hydrogen bonds in biological molecules. Although uncharged like gamma photons, neutrons interact much more strongly with air than gamma rays; thus, air can help shield against neutrons over moderate distances, particularly if saturated with water vapor. Neutron shielding strategies typically employ either: (a) relatively large thicknesses (e.g. 12″ to 36″ or more) and massive amounts of low-cost hydrogenous materials such as concrete or water; or (b) lesser thicknesses and smaller masses of higher-cost hybrid materials containing “dopants” comprised of elements having high neutron absorption cross sections (e.g. silicone sheets containing 25% Boron [B; Z=5] by weight; concrete containing depleted uranium oxide [U; Z=92; trademarked “DUCRETE”], etc.) and reduced propensity to emit prompt or delayed gammas after neutron absorption.        X-rays and gamma rays (no charge; electromagnetic photons with zero mass)—only effectively absorbed or attenuated by substantial thicknesses of denser materials such as ordinary concrete and/or lesser thicknesses of comparatively higher-Z materials such as steel (Fe: Z=26), lead (Pb: Z=82), tungsten (W; Z=74), or depleted uranium (U; Z=92) metal alloys. This will be discussed in more detail below.        
In order to have shielding with a high overall safety-factor, land-based commercial nuclear power reactors now typically utilize a welded steel containment vessel with walls ˜10″ thick; this is in turn surrounded by a containment building having steel-reinforced concrete walls at least 2-3 feet thick. By contrast, a proposed nuclear reactor design for long-duration manned space missions, such as to Mars or Jupiter's moons, utilized much less massive borated polyethylene radiation shields ˜three feet thick, primarily for shielding against prompt neutrons.
Attenuating (Shielding Against) Gamma Radiation:
Being electromagnetic radiation, gamma photons are attenuated exponentially; thus, it is not theoretically possible to design a shield that will stop 100% of all incident gamma radiation. As a result, gamma rays and hard X-rays are much more difficult to attenuate or shield against than charged particle radiation such as alphas or betas. Like neutrons, X-ray and gamma photons do not possess any charge. This absence of charge causes their level of interaction with normal matter to be reduced, which substantially increases their relative penetrating power compared to charged particles. Also, as gamma photon energies get higher, they are somewhat more difficult to attenuate; a 5 MeV gamma photon has more penetrating power than a 1 MeV gamma photon. When using readily available conventional materials, comparatively thick, heavy layers of steel, lead, tungsten alloys, depleted uranium, or conventional concrete are typically needed to effectively attenuate gamma radiation to acceptable levels. In general, as the density and/or thickness of a shielding material increases, the attenuation of incident gamma radiation by the material also increases. Generally speaking, the higher the atomic number (Z) of the shielding material, and/or the higher its density, the greater the degree of attenuation of gamma radiation.
Unlike the case with alphas, betas, and to a lesser extent neutrons, air is relatively transparent to gamma radiation and provides little or no shielding over distances that can range up to many hundreds of meters from intense, high-energy gamma sources.
The subject of attenuation of gamma and X-ray radiation utilizes two closely related concepts that are defined as follows:                Half-Value Layer (HVL)—is the thickness of any specified material necessary to reduce the intensity of an incident beam of gamma or X-ray photons to one-half (50%) of its original value. The thickness of the half-value layer for a specific material is a function of the energy of the incident gamma radiation and the elemental composition of the shielding material.        Tenth-Value Layer (TVL)—is the thickness of any specified material necessary to reduce the intensity of a beam of gamma or X-ray photons to one-tenth (10%) of its original value.        
The following Table 2 illustrates thicknesses of one half-value and one tenth-value layer for selected materials and two gamma sources often used in medical and industrial applications:
TABLE 2Approximate Thicknesses of One (1) Half-Value andOne (1) Tenth-Value Layer for 60Co and 137Cs(measured in inches)GammaOne (1) Half-ValueOne (1) Tenth-ValuePhotonLayerLayerGammaEnergyOrdinaryOrdinaryEmitter(MeV)LeadSteelConcreteLeadSteelConcrete60Co1.17; 1.330.47″0.83″2.6″ 1.6″2.7″8.2″137Cs0.6620.28″0.63″1.9″0.83″2.1″6.2″Data Source: U.S. Dept. of Labor - OSHA
Depending upon budgetary cost constraints, allowable physical volume of shielding, and total allowable mass of shielding, desired levels of gamma attenuation can be achieved by selection of specific shielding materials and thicknesses. For example, if a thickness of a given shielding material reduces the gamma flux to one-half of the incident value (i.e., a “half-value layer”), then the thickness of three such layers will reduce the dose to one-eighth (½×½×½) the initial amount; similarly, three “tenth-value layers” will reduce the dose to 1/1000 of the initial amount ( 1/10× 1/10× 1/10). Selected examples of “ 1/1,000 gamma shield” thicknesses are illustrated in Table 3 as follows:
TABLE 3Approximate Thicknesses of Three (3) Tenth-Value Layersfor60Co and 137Cs Attenuation of Gamma Flux to 1/1000of the Incident Radiation (measured in inches)Gamma3× One (1) Tenth-ValuePhotonLayerGammaEnergyOrdinaryEmitter(MeV)LeadSteelConcrete60Co1.17; 1.334.8″8.1″24.6″137Cs0.6622.5″6.3″18.6″
Relatively low-cost, readily available shielding materials such as steel, brick, concrete, water, or even packed earth can function as very effective gamma attenuators. Used properly, they can be quite cost-effective and provide the same degree of gamma shielding as higher performance materials if used in appropriately greater thicknesses in applications in which the total mass of shielding and/or its thickness are not important issues. This is why land-based nuclear reactors use thick layers of steel and concrete for shielding and containment. It is also the reason why manned military aircraft that are potentially exposed to gamma ray flashes from nuclear detonations currently have no real gamma shielding for pilots and crews. Although the U.S. military would willingly pay for the high cost of gamma shielding, existing high performance shielding materials cannot provide an effective shield with a mass that is low enough for both gamma attenuation and combat aircraft performance requirements to be met. Table 4 illustrates the approximate thickness (measured in inches) of one (1) tenth-value layer and three (3) tenth-value layers for 0.5 MeV and 0.8 MeV gamma photons for selected lower-cost, readily available materials:
TABLE 4Approximate Thicknesses of One (1) Tenth-Value Layerand Three (3) Tenth-Value Layers for Various Lower-CostMaterials Three (3) Tenth-Value Layers Attenuate GammaFlux to 1/1000 of the Incident Radiation (measuredin inches except where otherwise noted)One (1) Tenth-Value3× One (1) Tenth-LayerValue LayerGamma ShieldingGamma Photon EnergyMaterial0.5 MeV0.8 MeV0.5 MeV0.8 MeVLead0.55″ 1.0″ 1.7″ 3.0″Copper1.6″2.0″ 4.8″ 6.0″Iron (steel)1.9″2.3″ 5.7″ 6.9″Aluminum5.5″6.3″16.5″18.9″Concrete5.9″7.1″17.7″21.3″Packed Earth7.5″9.1″22.5″27.3″Water13.8″ 15.8″ 41.4″47.4″Air (measured951′ 1,115′  2,853′  3,345′  in feet)Data Source: Radiation Protection Manual at TRIUMF by Peter Garnett and Lutz Moritz, which utilized material from the book, Radiation Protection, Point Lepreau Generating Station by J. U. Burnham; see http://www.triumf.ca/safety/rpt/intro.html
FIG. 16 shows the thickness of one (1) half-value layer (measured in cm) of shielding as a function of incident gamma photon energy from 0.1 MeV up to 10.0 MeV for a shield composed of either air, water, concrete, aluminum, iron (steel), or lead.
In some applications, more costly, “high performance” shielding materials that have higher density and higher atomic numbers (e.g. lead or tungsten) may be preferable for use in gamma shields because less thickness and weight per square foot of shielding is required for them.
The following elements are known to be somewhat better performing gamma attenuators than lead: tantalum (Ta; Z=73), tungsten (W; Z=74), thorium (Th; Z=90; no stable isotopes—all are mildly radioactive, 232Th occurs naturally with half-life=1.4×1010 years), and depleted Uranium (called “DU”; used in military munitions: mostly 238U, Z=92; it is mildly radioactive).
Since DU and thorium are both slightly radioactive, their use may be barred in certain commercial shielding applications because of environmental and human health safety issues. Costs of less controversial higher performance gamma shielding materials can vary greatly. For example: as of this writing, the spot lead price in London is currently ˜$0.59/lb; the price of tungsten is now roughly $11.80/lb or ˜20× the price of lead; and current contract prices for tantalum are roughly $40-$50/lb or ˜68× to 85× the price of lead. At this time, lead is hard to surpass for cost-effectiveness in appropriate gamma shielding applications.
Gamma shielding requirements can be more demanding for applications involving persistent non-weapon physical environments in which very large numbers of neutron captures on various elements/isotopes are occurring. Such environments include:                Operating nuclear reactors (mostly thermal neutron captures),        “Target” materials exposed to various types of neutron beams (slow and fast neutron captures),        Containment vessels of proposed commercial versions of D-T fusion reactors (energetic neutrons produced as products of fusion reactions are captured by nearby materials), and        Surface and near-surface regions of highly-loaded hydrides or deuterides in which Low Energy Nuclear Reactions (LENRs) are catalyzed by Ultra Low Momentum Neutrons (ULMNs) captured on local nuclei. LENRs can be sustained over significant periods of time under specific types of non-equilibrium conditions satisfying certain key characteristics and parameters, such as a material's ability to support surface plasmon polaritons, high proton/deuteron flux across the surface, and so forth.        
Shielding for such neutron-rich nuclear environments can potentially be more difficult because (depending upon the isotopic composition of the neutron absorbers) neutron capture events frequently result in the production of prompt gammas, which tend to have higher average photon energies than delayed gammas produced in common nuclear decay chains (in which excess energy, Q, can often be distributed across a greater number of reaction products over a longer period of time). As illustrated in Table 4, higher gamma photon energies confer greater penetrating power and require significantly thicker shielding, all other things being equal.
An excellent database providing data on neutron capture gammas is available online as follows: “Database of Prompt Gamma Rays from Slow Neutron Capture for Elemental Analysis—Final report of a coordinated research project”                R. B. Firestone et al.        
International Atomic Energy Agency (IAEA), Vienna, Austria (2003)
Key sections/items found in the above prompt gamma ray database (which covers 395 normally abundant isotopes and nuclear isomers out of several thousand known isotopes) are as follows:                Table 7.3, pp. 94-158, “Adopted Prompt and Decay Gamma Rays from Thermal Neutron Capture for All Elements”—note especially that upon examination there are only three isotopes shown with gamma decay lines having energies>10.0 MeV as follows: 3He (20.520 MeV); 14N (10.829 MeV); and 77Se (10.496 MeV—see later note: the second highest-energy gamma line listed for 77Se is 9.883 MeV)        Table 7.4, pp. 159-177, “Energy-Ordered Table of Most Intense Thermal Neutron Capture Gamma Rays,”—note that in this Table, there are only three gamma lines>10.0 MeV are for: 3He (20.520 MeV); 14N (10.829 MeV); and 77Se (9.883 MeV—see note above; on 77Se, there is a discrepancy between Table 7.3 and Table 7.4)        
An examination of Tables 7.3 and 7.4 from the above reference yields the following:                Except for the three isotopes noted above (3He, 14N, and 77Se), virtually all of the other neutron capture gammas listed in the IAEA database have photon energies that fall below 10.0 MeV.        A high percentage of prompt gammas from slow neutron capture have energies that fall between 0.5 MeV and 10.0 MeV. Many of these have many multi-MeV spectral lines.        Neutron capture on a proton produces a deuteron plus a 2.24 MeV gamma        Neutron capture on deuterium produces a 6.250 MeV gamma        Neutron capture on 3He produces a 20.520 MeV gamma        
There can be significant qualitative and quantitative differences between delayed gamma spectra of various isotopes and prompt (especially fission) gamma spectra. FIG. 17 illustrates an essentially “pure” prompt gamma spectrum resulting from a fissile isotope: the y-axis on the chart is gamma photon flux (cm−2·MeV−1 for one fission); the x-axis is photon energy in MeV. The data underlying this Figure was reconstructed from extensive data automatically collected during a serious criticality “gamma flash” incident triggered by a freak handling accident with a critical assembly consisting of a highly enriched uranium (HEU) core and predominantly copper reflector. The incident resulted in one fatality, occurred at the Nuclear Center, in Sarov, Russia, in 1997 and was investigated by the International Atomic Energy Agency.
For the purpose of discussing FIG. 17, highly enriched uranium (HEU) means 238U that has been enriched up to >20% 235U. HEU is used in nuclear weapons and some types of military reactors, not in civilian uranium-based power reactors, which use only 3%-5% 235U. 235U is a fissile isotope, which means that it can support a runaway chain reaction.
Quoting from the IAEA accident report, “ . . . a component from the upper reflector slipped from the technician's rubber gloved hand and fell into the lower part of the assembly, which . . . contained the enriched uranium core. The point of criticality was exceeded, there was a flash of light and a wave of heat, and the lower part . . . was ejected downward . . . ”
In FIG. 17 it is obvious by visual inspection of the respective total areas under the curve that the total flux of gamma photons between 0.5 and 10.0 MeV is substantially larger than the total flux between 0 and 0.5 MeV. This is consistent with the inventors' qualitative observation with respect to the IAEA “Database of Prompt Gamma Rays from Slow Neutron Capture for Elemental Analysis” in the bulleted points noted above.
The prompt gamma spectrum illustrated in FIG. 17 comes mostly from the fission of 235U, and to a lesser extent from prompt gammas produced by fast neutron captures on 238U, copper (63Cu and 65Cu), and whatever other elements happened to be present in the critical assembly's “predominantly copper” reflector. This spectrum is similar to what would be produced during the initial prompt gamma “flash” in the detonation of a typical uranium-based fission weapon.